PostCSS Configuration Is a Function

Why This Error Occurred

The project's custom PostCSS configuration exports a function instead of an object.

Possible Ways to Fix It

Adjust the custom PostCSS configuration to not export a function. Instead, return a plain object.

If you need environment information, read it from process.env.

Before

module.exports = ({ env }) => ({
  plugins: {
    'postcss-plugin': env === 'production' ? {} : false,
  },
})

After

module.exports = {
  plugins: {
    'postcss-plugin': process.env.NODE_ENV === 'production' ? {} : false,
  },
}